Systems and methods for providing overcharge protection in capacitive coupled biomedical electrodes

ABSTRACT

An alternating electric field responsive biomedical composite is disclosed that provides capacitive coupling through the composite. The biomedical composite includes a binder material, a polar material that is substantially dispersed within the binder material, and electrically conductive particles within the binder material. The polar material is responsive to the presence of an alternating electric field, and the electrically conductive particles are not of sufficient concentration to form a conductive network through the composite unless and until the composite becomes overcharged.

BACKGROUND

The invention generally relates to conductive and non-conductivematerials that are used in conjunction with providing an electric fieldat one side of such a material responsive to an electric field on theother side of the material for biomedical applications.

The design of an electrically conductive pressure sensitive adhesive(PSA) for biomedical applications has long presented challenges at leastbecause adhesive strength and flexibility generally decrease withincreased electrical conductivity. The materials that are typically used(e.g., added) to provide good electrical conductivity are generally lessflexible and inhibit adhesion. A conventional way to prepare aconductive coating is to fill a polymeric material with conductiveparticles, e.g., graphite, silver, copper, etc., then coat, dry and curethe polymeric binder. In these cases, the conductive particles are insuch a concentration that there is a conductive network formed when theparticles are each in physical contact with at least one otherneighboring particle. In this way, a conductive path is provided throughthe composite.

For pressure sensitive adhesives (PSAs), however, if the particleconcentration is high enough to form a network in whichparticle-to-particle contact is maintained then there is little chancethat the polymer (e.g., elastomer) system of the PSA component ispresent in high enough concentrations to flow out to makesurface-to-surface contact between the substrates and an electrode,i.e., act as an adhesive. Conversely, if the PSA component is insufficient concentration to make sufficient surface contact to thesubstrate, it would have to interrupt adjacent conductive particles suchthat particle-to-particle contact is disrupted.

Another type of electrically conductive PSA includes conductivespherical particles with diameters equal to or greater than thethickness of the PSA. In this fashion the signal or current may becarried along the surface of the particles, thus providing current flowanisotropically in the z dimension of the adhesive. Such a composite hasnot been shown in the prior art to be usable for a biomedical adhesive.

Salts, such as sodium or potassium chloride, readily dissolve when in anaqueous medium, and their ions dissociate (separate into positive andnegative ions). The dissociated ions may then convey an electricalcurrent or signal. For this reason, salts have long been added to water,which then may be added to polymeric and elastomeric materials, toprovide good electrical conductivity. For example, U.S. Pat. No.6,121,508 discloses a pressure sensitive adhesive hydrogel for use in abiomedical electrode. The gel material is disclosed to include at leastwater, potassium chloride and polyethylene glycol, and is disclosed tobe electrically conductive. U.S. Pat. No. 5,800,685 also discloses anelectrically conductive adhesive hydrogel that includes water, salt, aninitiator or catalyst and a cross linking agent. The use of suchhydrogels however, also generally requires the use of a conductivesurface at one side of the hydrogel (away from the patient) that iscapable of receiving the ionicly conductive charge, such assilver/silver chloride, which is relatively expensive.

While these hydrogel/adhesives can have good electrically conductiveproperties, they often have only fair adhesion properties. Anotherdownside is that the electrical conductivity changes with changing watercontent, such as changes caused by evaporation, requiring that thehydrogels be maintained in a sealed environment prior to use, and thenused for a limited period of time due to evaporation.

U.S. Pat. No. 7,651,638 discloses a water insensitive alternatingcurrent responsive composite that includes a polymeric material and apolar material (such as a salt) that is substantially dispersed withinthe polymeric material. The polar material however, is not employed toprovide electrical conductivity via ionic conduction. The polymericmaterial and the polar material are chosen such that the two materialseach exhibit a mutual attraction that is substantially the same as theattraction within the individual materials. Because of this, the polarmaterial neither clumps together nor blooms to a surface of thepolymeric material, but remains suspended within the polymeric material.This is in contrast to the use of these salts in other applications thatare intended to bloom to a surface to provide a conductive layer along asurface, e.g., for static discharge.

The composites of U.S. Pat. No. 7,651,638, however, remain dielectricsand have high resistance, and are therefore not suitable for use incertain applications, such as providing electrical stimulus to a humansubject (defibrillation and/or transcutaneous electrical nervestimulations, etc.) due to the high resistance of the material. Thistype of signal detecting adhesive is also not capable of dissipating thecharge overload in a timely enough fashion as per AAMIEC12-2000-4.2.2.4, which is directed to defibrillation overload recovery(DOR). The materials are therefore not suitable for use as a monitoringelectrode through which a signal may be needed to be detected after adefibrillation charge is applied to a patient. The failure to pass AAMIEC12-2000-4.2.2.4 is due to the high impedance of these capacitivelycoupled adhesives.

There remains a need, therefore, for a composite for use in conducting arepresentative signal and/or current through at least the z dimension ofa PSA in a biomedical electrode, such that the use of conductiveparticles may be minimized, while preserving the adhesive's properties,so that both good electrical performance and good adhesive propertiesmay be maintained.

SUMMARY

The invention provides an alternating electric field responsivecomposite for use in a biomedical electrode that provides capacitivecoupling through the composite in accordance with an embodiment. Thecomposite includes a binder material, a polar material that issubstantially dispersed within the binder material, and electricallyconductive particles within the binder material. The polar material isresponsive to the presence of an alternating electric field, and theelectrically conductive particles are not of sufficient concentration toform a conductive network through the composite, yet will provide anovercharge protection in the event, for example, of a defibrillationprocedure.

In accordance with an embodiment, the overcharge protection is providedby having the electrically conductive particles migrate viaelectrophoresis to form electrically conductive paths through thecomposite.

In accordance with another embodiment, the binder material and the polarmaterial exhibit mutual molecular compatibility, and the electricallyconductive particles remain substantially isolated from one anotherwithin the binder material.

In accordance with a further embodiment, the electrically conductiveparticles may be carbon or graphite in the form of powder, flakesgranules, nanotubes, etc.

In accordance with a further embodiment, the invention provides a methodof providing overcharge protection in a biomedical electrode usingelectrophoresis.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description may be further understood with reference tothe accompanying drawings in which:

FIG. 1 shows an illustrative diagrammatic view of a composite inaccordance with an embodiment of the invention prior to electrophoresis,and FIG. 1A shows an enlarged view of a portion thereof;

FIG. 2 shows an illustrative diagrammatic view of the composite of FIG.1 in the presence of a rising biomedical electric field (V_(bio+)), andFIG. 2A shows an enlarged view of a portion thereof;

FIG. 3 shows an illustrative diagrammatic view of the composite of FIG.1 in the presence of a falling biomedical electric field (V_(bio−)), andFIG. 3A shows an enlarged view of a portion thereof;

FIG. 4 shows an illustrative diagrammatic view of the composite of FIG.1 in the presence of an overcharge electric field (V_(overcharge)), andFIG. 4A shows an enlarged view of a portion thereof;

FIGS. 5A-5C show illustrative diagrammatic views of the composite ofFIG. 1 at successive moments after a direct current (DC) overchargeelectric field is applied showing the electrophoresis activity;

FIGS. 6A-6B show illustrative diagrammatic views of the composite ofFIG. 1 at successive moments after an alternating current (AC)overcharge electric field is applied showing the electrophoresisactivity;

FIG. 7 shows an illustrative diagrammatic view of a composite of thepresent invention following application of an overcharge electric fieldover a common overcharge area;

FIG. 8 shows an illustrative diagrammatic view of a wide area of acomposite of the present invention showing the selectively anisotropicnature of the composites of the invention providing the multiplediscontinuous overcharge areas may be formed;

FIG. 9 shows an illustrative diagrammatic view of a composite inaccordance with a further embodiment of the invention, and FIG. 9A showsan enlarged view of a portion thereof;

FIGS. 10A and 10B show illustrative graphical representations ofbiomedical sensor output date in a conventional anisotropic measurementdevice, and in unitary a composite in accordance with an embodiment ofthe invention respectively;

FIGS. 11 and 12 show illustrative micro-photographic views of compositesof the invention at different magnifications;

FIGS. 13-16 show illustrative diagrammatic views of biomedicalelectrodes in accordance with various embodiments of the invention; and

FIGS. 17-18 show illustrative diagrammatic views of composites of afurther embodiment of the invention employing carbon nanotubes beforeand after electrophoresis.

The drawings are shown for illustrative purposes only and are not toscale.

DETAILED DESCRIPTION

Applicants have discovered that although the composites of U.S. Pat. No.7,651,638 are disclosed to function by capacitive coupling, conductiveparticles may be added to such composites with surprising results;although they are not added in such quantity that they form a conductivenetwork, the electrically conductive composites undergo electrophoresiswhen the composite is exposed to an overcharge voltage such as, forexample, the 200 volts DC as used in AAMI EC12-2000-4.2.2.2.4. Such anovervoltage charge would occur if a defibrillation procedure wasperformed on a patient being monitored. Failure to dissipate the chargefrom the electrode in a timely enough fashion so that the electrodes canagain pickup ECG signals, may result in additional defibrillationprocedures being done due to the absence of an ECG signal. Further, acapacitive discharge from the electrode to the patient may cause burnsto the patient's skin.

It has been found however, that the electrically conductive particles,when in the presence of the overcharge voltage, migrate within thebinder so as to form independent conductive paths through the composite,thereby causing the resistivity through the composite to dropsignificantly. This functionality provides an overcharge protection tothe biomedical electrode.

The impedance may be measured by the method described in AAMIEC12-2000-4.2.2.1 (AC Impedance), which provides a maximum of 3000 Ohmspermitted for any single value and an average not to exceed 2000 Ohms.The AC impedance method used herein was modified to 20 Hz rather than 10Hz, using a QuadTech 1920 Precision LCR meter sold by QuadTech, Inc. ofMarlborough, Mass.

It has been found however, that examples of composites of the inventionincluding just 5% by weight carbon particles have resistances of lessthan 1000 Ohms following overcharging, meaning that the composites passAAMI EC12-2000-4.2.2.4, yet function by the capacitive couplingtechniques disclosed in U.S. Pat. No. 7,651,638 before being subjectedto an overcharge electric field. It has further been found, in fact,that by adding as little as (1%) of a conductive particle eitherrandomly dispersed or position specific within a polymeric materialincluding a polar material as described above, composites may be formedthat pass AAMI EC12-2000-4.2.2.1 and AAMI EC12-2000-4.2.2.4 followingovercharging. Lower resistance mixtures (following overcharging) wereobtained using a 2.5% conductive particle addition, and still lowerresistance mixtures (following overcharging) were obtained using a 5%conductive particle addition.

A further aspect of the present invention is that since therepresentative signal from the aligning/relaxing electric fields of thepolar material is present in the z direction, a large area (in the x andy directions) material may be employed that contains multiple receivercontacts on the common large area material. The material, therefore, isanisotropic in that sensor contacts may be adjacent one another on thecommon composite material without cross signal detection. Moreover, thecomposite material remains anisotropic following overcharging since theconductive paths formed by electrophoresis are discrete from one anotheras discussed further below.

The requirements for the binder material (e.g., polymeric material orelastomeric material), the polar material and the conductive materialare that the materials interact in such a way that neither the polarmaterial nor the conductive material clumps together within the bindermaterial or blooms to a surface of the binder material. If theconductive material has a surface energy similar to that of the bindermaterial, then it will remain suspended within the binder material yetnot be in sufficient concentrations to provide electrical conductivitythrough the material prior to any overcharging.

FIG. 1 for example, shows a composite 10 in accordance with anembodiment of the invention that includes a binder material 12 andconductive particles 14 dispersed within the binder material 12. Asshown at the diagrammatic enlarged view 16 in FIG. 1A, the bindermaterial 12 includes a polymeric material 18 and a polar material 20that are combined at a molecular scale. This may be achieved, forexample, by introducing the polar material (while in an evaporativewater/alcohol solution) into the solvated and/or liquid dispersedpolymeric material and then permitting the water/alcohol solution toevaporate leaving the polar material within the polymeric material.

In accordance with an embodiment of the invention, the polymericmaterial may, for example, be an acrylic adhesive such as may berepresented as

where R may vary and may be any of an ethyl, or a butyl or a2-ethylhexyl or other organic moiety, and n is a number of repeatingunits. For example, the polymeric material may be a FLEXcon V95 pressuresensitive adhesive as sold by FLEXcon Company, Inc. of Spencer, Mass.

In an embodiment, the polar material may be a quaternary ammonium saltsuch as may be represented as:

where R=H or some carbon based moiety, and where any of the R groups maybe the same or different. For example, the polar material may be anArquad HTL8-MS quaternary ammonium salt sold by Akzo Nobel Surfactantsof Chicago, Ill.

An objective of the selection of the combination of the binder materialand the polar material is that the two materials each exhibit a mutualattraction that is very similar to the attraction that each material hasto its own molecules. This results in the polar material beinghomogeneously dispersed within the binder material. The suitability ofthe combination of the polymeric material and the polar material may beidentified by the following procedure. First, a polar material iscombined with the polymeric material in about five differentconcentrations (typically between about 5% to about 45% by weight). Thenthe adhesive and salt composite is drawn onto a release liner (of about1.5 mil), and permitted to dry and cure. The surface of the composite isthen inspected after a short period of time. If the polar material hascrystallized out or bloomed to the surface, then the combination ofcomponents is not compatible. If, on the other hand, the composite isclear, it is subjected to the next level of compatibility testing. Thesamples should then be subjected an exposure test in which the samplesare exposed to 100 F with 95% relative humidity for 3 days. The samplesare then again inspected to determine whether the polar material hasmigrated toward either surface. If there has been no migration of thepolar material and the composite is clear, then the dielectric constantfor the composite is determined and the composite is tested for use as amedical monitoring material.

With reference again to the diagrammatic enlarged view 16 of FIG. 1, thebinder material and the polar material are selected to be compatible butnot such that they undergo an ionic disassociation change such as wouldoccur, for example with NaCl in water. The molecule-scale polar material20 is therefore dispersed within the binder material 18 but given themolecular weight of the polar material and the non-protic medium of theadhesive, little or none true ionic disassociation would be expected.

As shown in FIG. 2, when an external positive bio-electric field(V_(bio+) as generally shown at 22) is present at one side 24 of thecomposite 10, as shown in the enlarged view of FIG. 2A, the polarmaterial 20 responds by aligning with the external positive electricfield as shown in the enlarged view. As shown in FIG. 3, when theexternal bio-electric field decreases (V_(bio−)) at the one side 24 ofthe composite 10, as shown in the enlarged view of FIG. 3A, the polarmaterial 20 is free to migrate to random orientations. When this occurs,a positive charge (V_(out)+) is provided at a second opposite surface 26of the composite 10. Upon the bio-electric field collapsing, the polarmaterial under normal thermal motion, returns to a random state. Thereleased electrical potential may be detected by an electrode as shownat 28. As the bio-electric field alternates therefore from V_(bio−) toV_(bio+) to V_(bi−) etc., the output signal provides a representativealternating signal of V_(out+) to V_(out−) to V_(out+) etc.

The voltage at the electrode on the surface 26, therefore, alternates inthe presence of an alternating electric field at the opposite surface24. In this way, an alternating electric field from the first side ofthe composite may be represented by a second alternating electric fieldprovided at the electrode 28. Note that the capacitance may varydepending upon the size (e.g., the X-Y plane and the total distancebetween conductive surfaces.

The conductive particles should have a surface energy that is at leastslightly greater than that of the binder material to ensure that thebinder material sufficiently wets the surface of the conductiveparticles. The density and surface area of the conductivity of theparticles 14 are important considerations. Applicants have found, forexample, that carbon (e.g., graphite powder, flakes, granules ornanotubes etc.) having densities in the range of, for example, about0.35 g/cm³ to about 1.20 g/cm³, and preferably between about 0.5 g/cm³to 1.0 g/cm³, are suitable for use as the conductive material. Thesurface energy of the graphite is, again, preferably higher than that ofthe binder to ensure sufficient wetting of the surfaces of the particles14. In the above example, the graphite particles have a specific surfaceenergy of 55 dynes/cm and the binder disclosed above has a surfaceenergy of less than 40 dynes/cm.

FIG. 4 shows the composite of FIG. 1 in the presence of an overvoltagecharge 30 (V_(overvoltage)). As shown, in the presence of such anovervoltage charge 30 on an electrode 32, as shown in the enlarged viewof FIG. 4A, the conductive particles 14 align with the shortest distancebetween a high charge and a low charge (such as ground) due to anelectrophoresis process. The aligned conductive particles therebymigrate to form a permanent conductive path through the composite asshown. The overvoltage charge may now conduct along the path formed bythe conductive particles 14.

In particular, FIGS. 5A-5C show the electrophoresis process that occursupon overcharging in more detail. As shown in FIG. 5A, when a largevoltage potential is applied, e.g., 5, 10, 50, 100 or 200 volts orhigher AC or DC, a particle 14 a that is near the surface aligns in thez-direction. Once this occurs, the inner end 16 a of the particle 14 ais now closer to the opposing surface, causing the charge on the innerend 16 a to be slightly higher than the charge on the surrounding innersurface of the composite. This causes another nearby particle 14 b to beattracted to the inner end 16 a of the particle 14 a as shown in FIG.5B. The inner end of the particle 14 b is now highly charged, causingnearby particle 14 c to be attracted to it as shown in FIG. 5C. Furtherparticles (e.g., 14 d as shown) are further attracted to the ends of thethus formed path. This all occurs rapidly and the attractive/aligningforce causing the electrophoresis is believed to become stronger as thepath is formed as the distance between a first electrode and the growingagglomerate attached to the other electrode gets smaller.

As shown in FIG. 6A, when an AC overvoltage field is applied (again,e.g., 5, 10, 50, 100 or 200 volts or higher), the particles 15 a and 15b form along a first side of the composite 12 that has a positivevoltage applied to it at a first conductor 31. When a positive voltagecharge is then applied at the opposite conductor 33, the conductiveparticles 15 c and 15 d then begin to agglomerate from the lower side ofthe composite as shown in FIG. 6B. By thus alternating the agglomerationprocess between opposite sides, the AC overvoltage causes a path to beformed that essentially meets in the middle.

Regardless of whether the overvoltage charge is DC or AC, the higher thevoltage, the faster the particles align, but with a relatively lowvoltage (e.g., about 5 volts or higher), the particles align moreslowly, but do still eventually align.

As shown in FIG. 7, following overcharging over a small area of thecomposite, multiple conductive paths 34 may be formed through thecomposite, wherein each conductive path is formed by aligned conductiveparticles. As shown in FIG. 8, groups of such conductive paths 38, 40,42 may be separated from one another through selective application ofovercharging fields, permitting selected areas of the composite to beelectrically conductive, while other areas 36 of the composite exhibit ahigh dielectric constant and are therefore not electrically conductive.

In accordance with a further embodiment, a composite 50 of the inventionmay include a first portion 52 that exhibits capacitive coupling asdescribed above, while another portion 54 of the composite includesconductive paths, e.g., formed of spheres 56 of carbon, that extend justslightly through the binder material as shown in FIG. 9 as well as theenlarged view thereof of FIG. 9A. Such as composite may be used toselectively provide capacitive coupling in one area 52 (as discussedabove with reference to FIGS. 1-3) and/or to provide electricalconductivity in a different area 54.

In accordance with an embodiment, in one example, a polar material(Arquad HTL-8 (AkzoNobel), 20% by weight on solids) was added to aliquid sample of FLEXcon's V-95 acrylic PSA. To this, 5% by weight(solids of the V-95 FLEXcon and Arquad blend) of a carbon particle (theAquablack 5909 carbon particles from Solution Dispersions Inc.,Cynthiana Ky.), was uniformly dispersed. This mixture was coated onto a2 mil (50 micron) siliconized one side PET film, dried and cured for 10min in a 160° F. vented laboratory oven, to a dried deposition of 2 mil(50 micron).

It has been found that after performing the test procedure in AAMIE12-2000-4.2.2.4 the adhesive composite with the conductive particlesdispersed within undergoes a change. Post device overload recovery (DOR)tested materials will now pass AAMI E12-2000-4.2.2.1. It has also beenfound that like the capacitively coupled binder material, which has Zdimension signal receptivity, the post DOR material maintains this Zdimension signal receptivity. In addition the conductive particlevariant, post DOR test, material also conveys current in the Zdimension. Interestingly this maintenance of Z dimensionality allowsthis adhesive to be used in applications as disclosed in U.S. PatentApplication Publication No. 2010-0036230 which teaches the formation ofa bio-sensor array fashioned with one continuous layer of adhesive, thedisclosure of which is hereby incorporated by reference in its entirety.

Composites in accordance with certain embodiments of the presentinvention, begin with substantially separated particles uniformlydispersed within the adhesive, then requires a second step, i.e.,applying an electric field to form the conductive structures. This is adecided advantage as it allows for the placement of conductivestructures, i.e., in the Z dimension and if needed, place the Zdimensioned structures at specific X,Y, locations thus allowing for aspecific point to point electrical contact.

The following Examples demonstrate the effect of the conductive particleaddition to the binder material discussed above.

Example 1

To a liquid sample of FLEXcon's V-95 acrylic PSA, is added the polarmaterial, Arquad HTL-8 (AkzoNobel), 20% by weight on solids, to this 5%by weight (solids of the V-95 and Arquad blend) of a carbon particle(Aquablack 5909 from Solution Dispersions Inc., Cynthiana Ky.), wasuniformly dispersed and designated as Sample 1. This mixture was coatedon a 2 mil (50 micron) siliconized one side PET film, dried and curedfor 10 min in a 160° F. vented laboratory oven, to a dried deposition of2 mil (50 micron).

Also prepared at this time was the composite of just the V-95 acrylicadhesive and the Arquad (20% by solids weight), no carbon, as per theteachings in U.S. Pat. No. 7,651,638. This mixture was also 2 mil (50microns) siliconized one side PET film, dried and cured for 10 min in a160° F. vented laboratory oven, to a dried deposition of 2 mil (50microns) and was designated as Sample 2.

Similarly a third sample was prepared consisting of only V-95 arcylicadhesive and 5% carbon, with no polar material (Arquad). The sample wasprocessed in the same manner as for samples 1 and 2, and this sample wasdesignated as Sample 3.

All three samples were tested on a conductive base material consistingof a carbon filled polymeric film with a surface resistance of ˜100ohms/square, using the experimental product designated EXV-215, 90 PFW(as sold by FLEXcon Company, Inc. of Spencer, Mass.). The samples weretested using a QuadTech LCR Model 1900 testing device as sold byQuadTech, Inc. of Marlborough, Mass.

In particular, all three samples were then tested as per AAMIEC12-2000-4.2.2.1 (modified) and AAMI EC12-2000-4.2.2.4. The AAMIEC12-2000-4.2.2.1 test has an upper limit of 3000 Ohms for the face toface double adhesive part of the test, for a single point and a maxaverage (12 test samples) of 2000 Ohms.

The AAMI EC12-2000-4.2.2.4 calls for retaining less than 100 mV in 5 secafter a 200 DC volt charge, again using a face to double adhesive layer.

Note the Table 1 below, which shows impedance (EC 12-2000-4.2.2.1)tested first; DOR (EC 12-2000-4.2.2.4) was run next on the same samples.

TABLE 1 EC12-2000- EC12-2000- Sample 4.2.2.1 (20 Hz) 4.2.2.4 Sample 160K 0.0 volts in less Ohms (fail) than 5 sec. (pass Sample 2 80K 150volts after 5 Ohms (fail) sec. (fail) Sample 3 40M 0.0 volts in lessOhms (fail) than 5 sec. (pass)

Example 2

To determine the signal receptivity of this invention, the samplesprepared for Example 1 were tested in accordance to the procedureoutlined below. The samples used in testing as per AAMIEC12-2000-4.2.2.1 were used connected in series to a Wave Fowl Generator(Hewlett Packard 33120A 15 MHz Function/Arbitrary Waveform Generator)and in series an Oscilloscope (BK Precision 100 MHz Oscilloscope 2190),schematically shown below. Samples were tested at 3, 10 and 100 Hz;results are given below in Table 2 in % of transmitted signal received.

TABLE 2 Sample 1 Sample 2 Sample 3  3 Hz 95+% 95% No signal  10 Hz 95+%95% No signal 100 Hz 95+% 95% No signal

Example 3

Samples that passed the DOR test (AAMI EC12-2000-4.2.2.4) were retestedfor impedance as per AAMI EC12-2000-4.2.2.1 (modified), upon rechecking,samples 1 & 3 had a remarkable change. Samples 1 and 3 now had animpedance of less than 1K Ohms. In sample 2, the signal receptive mediumwas unchanged post DOR test; only those samples with the dispersedconductive particles changed. Further, the resulting lower impedance wasstill anisotropic, i.e., in the Z direction (noting Example 4 as to howthe anisotropic property was determined). In addition the parallelcapacitance (CP) of the post DOR material actually increases as the Zimpedance decreases, as shown below in Table 3.

TABLE 3 Ohms CP DC Resistance (Z direction) Farads Ohms Sample 1 60K11.0 nF 80K pre-DOR Sample 1 860 61.6 nF 790 post-DOR Sample 3 13M 0.06nF 100⁺M pre-DOR Sample 3 1.9K 41.2 nF 1.45K post-DOR

Example 4

The anisotropic property was validated by the following test procedure.Signals at 3, 10, 100, Hz were generated, and fed to a first coppershunt, which was placed on the conductive adhesive. A second coppershunt was placed on the same conductive adhesive ˜0.004″ (100 microns)apart from the first shunt, which was connected (in series) to anoscilloscope. The base substrate was a dielectric material (PET film).Following electrophoresis therefore, the composite may have a resistanceof less than about 3,000 Ohms, less than about 2,000 Ohms, less thanabout 1,000 Ohms or even less than about 500 Ohms.

If the Sample 1 adhesive was isotropic it would have been expected topick up a signal on the oscilloscope. If the Sample 1 adhesive wasanisotropic it would have been expected that no signal would be receivedon the oscilloscope. The result was that no signal was detected.

FIG. 10A shows at 60 a set of ECG test recordings using conventionalbiomedical sensors, and FIG. 10B shows at 62 the same set of ECG signalsrecorded using a biomedical sensor in accordance with an embodiment ofthe invention.

The electrophoresis result does not appear to rely on the presence ofthe polar material in the composite. It is believed that the carbonparticles are agglomerated by the electric field applied during the DORtest; that the electric field generated by the 200 DC volts beingapplied across the conductive particle containing SRM and/or theconductive particles just with a PSA (no polar organo-salt), issufficient to cause the particles to agglomerate together.

The agglomerated structures spanning from one electrode to the other arethe reason an anisotropic conductive PSA is formed. To examine theseagglomerations, reference is made to FIG. 11, which shows at 70 an insitu formed conductive structure as per this invention. In particular,FIG. 11 shows a 10× view looking down from the top of a conductivestructure. The dark areas are the agglomerated particles the lighterarea represents particle poor areas, i.e., places from which particlesmigrated.

This particle migration effect can be shown in more detail by looking atFIG. 12, which shows at 72 a 100× magnification of a conductivestructure, again looking down from the top but focused more towards theedge, showing the lighter, particle poor areas. The clear material isthe continuous medium, in this case a PSA, FLEXcon's V-95 acrylicadhesive. Note the striations or grooves in the clear V-95, and alsonote that the few particles remaining are aligned with the striations.The starting material was a uniform particle distribution in continuousmedium, thus under the electric field generated by the DOR test, theparticles move together to form the conductive structure. Again, thisagglomeration phenomenon may be referred to as electrophoretic or in thecase of an AC electric field, dielectrophoretic effect, both of whichare referred to herein as an electrophoresis process.

It is significant, however, that in this case the agglomeration occursin a non-aqueous high viscosity medium. In accordance with the presentinvention, the continuous medium is a dielectric and is in full contactwith the conductive particles (at the particle loading levels) and themedium is a viscoelastic liquid, i.e., has a very high viscosity, fivetimes plus orders of magnitude higher (as measured in centipoises) thanwater dispersions (often measured in the only the 10 s of centipoises).

Again, what is postulated here is that, as in the case of particleagglomeration via an overcharge electric field in an aqueous continuousmedium, a slight charge is induced on a nearby particle near anelectrode. With the continuous medium being less polar and moredielectric than water however, a greater charge build-up can occur on aparticle in the electric field.

With water as the continuous medium the higher polarity would mitigatethe charge build up, further if the applied electric field wereincreased (higher voltage) electrolysis of the water would become acompeting complication. With a PSA (FLEXcon's V-95 acrylic adhesive) asthe continuous medium there is much less charge mitigation and nosubstantial electro-chemical process that occurs.

This charge build-up on the particle increases the attractive forcesbetween the particle and the electrode, thus drawing the particle to theelectrode in spite of the higher viscosity of the continuous medium.Further, the first particle that reaches the electrode forms anincremental high spot on said electrode thus the electric field is movedcloser to the other electrode, as more particles join the agglomerationthe field strength increase as the distance to the opposite electrodedecreases, thus accelerating the agglomeration growth.

The DOR test involves a plane to plane electrode arrangement; after afew conductive structures are formed therefore, the electric fieldbetween the two electrodes is mostly dissipated due to the contactsalready made between the electrodes. Thus the first structure will form,where there is one spot where the two planes are closer to one anotheror there is an uneven distribution of carbon such that a slightly higherdensity of the conductive particles are at one increment, between theplane, in other words that point of least resistance.

As a result using the plane to plane method in forming these structureshas some limits as to the position and number of conductive structuresformed. When a point-to-plane or point-to-point method is used tointroduce the electric field however, more discrete in position andnumber of conductive structures would be formed as each point has itsown electric field which is not readily dissipated when nearbyconductive structures are fanned.

This was demonstrated by using a lab corona treating device on aconductive substrate that was grounded. The lab corona treating deviceacted like a series of point sources to a plane receiving substrate.What resulted was a uniformly distributed conductive structure acrossthe surface of the adhesive.

The testing of the stability of the in situ formed electricallyconductive structures was accomplished by placing post DOR test samplesin an oven at 160° F. (71° C.) for 16 hours and retesting the impedance(AAMI EC12-2000-4.2.2.1.) and signal receptive properties. In all casesthe samples maintained the lower impedance.

The invention therefore provides that an overload protected capacitivelycoupled, water content insensitive, composite may be provided thatincludes a polymer and a polar material dispersed therein, andelectrically conductive particles, such that in the event ofovercharging, the impedance as measured by AAMI EC12-2000-4.2.2.1becomes less than 3,000 Ohms. The conductive particles may be in theform of carbon, and may be provided in a concentration greater than 1%on solids, dry weight. The composite may be anisotropic, and the polymermay be a pressure sensitive adhesive for use in an ECG electrode thatsatisfies the standards of AAMI EC-12-2000-422.4 for overload recovery

FIG. 13 shows a biomedical electrode 80 in accordance with an embodimentof the invention that includes a composite 82 of the invention includinga polymer, a polar material as discussed above, and conductiveparticles. Biomedical signals from the underside of the electrode (asshown at 88) may be picked up by, for example, a snap connector 84 thatis potted within a further supporting material 86 such as anotherpolymeric material. Note that bioelectric signals that are not directlyunder the snap connector (as shown at 89) are not picked up by the snapconnector 84.

FIG. 14 shows a biomedical electrode 90 in accordance with anotherembodiment of the invention that includes a composite 92 of theinvention again, including a polymer, a polar material as discussedabove, and conductive particles. A conductive layer 94 is formed on oneside of the composite 92 to provide that biomedical signals from theentire underside of the electrode (as shown at 99) may be picked up by,for example, a snap connector 96 that is potted within a furtherpolymeric material 98 as discussed above.

The use of the composite 92 of the present invention, however, providesthat the conductive layer 94 does not need to be formed of an expensivematerial such as silver/silver chloride (Ag/AgCl) as is required withhydrogels. Hydrogels require such specialized conductive layers becausethe ionic conductivity of the hydrogel must ionically couple to theelectrode. In accordance with the present invention on the other hand,the conductive layer 94 may be fanned of an inexpensive deposited layer(e.g., vacuum deposited or sputter coated) of, for example, conductiveparticles such as those discussed above but in a much higherconcentration to form a conductive layer upon deposition. Such lessexpensive materials may be used for the conductive layer because themechanism for conduction (whether by the polar material or theconductive material) is not ionic conductivity.

The use of inexpensive materials for the conductive layer also permitsthat a variety of connection options may be provided on a singlebiomedical electrode. For example, FIG. 15 shows a biomedical electrode100 in accordance with another embodiment that includes a composite 102of the invention again, including a polymer, polar material as discussedabove, and conductive particles. An extended conductive layer 104 isformed on one side of the composite 102 to provide that biomedicalsignals from the entire underside of the electrode (as shown at 112) maybe picked up by, for example, a snap connector 106 that is potted withina further polymeric material 108 as discussed above and/or a tabconnector 110 having an exposed portion 114 of the conductive layer 104.

FIG. 16 shows a biomedical electrode 120 in accordance with anotherembodiment that includes a composite 122 of the invention again,including a polymer, polar material as discussed above, and conductiveparticles. An extended conductive layer 124 is formed on one side of thecomposite 122 to provide that biomedical signals from the entireunderside of the electrode (as shown at 132) may be picked up by, forexample, a snap connector 126 that is potted within a further polymericmaterial 128 as discussed above and/or a tab connector 130 having anexposed portion 134 of the conductive layer 124.

Composites of further embodiments of the invention may employ carbonnanotubes. Such composites also undergo the electrophoresis processdiscussed above during overcharging, but the agglomeration results in ajumbled nest of the nanotubes given the very high aspect ratio of thenanotubes (e.g., upwards of 1000 to 1). For example, a composite 150 mayinclude carbon nanotubes 152 dispersed within a binder material 154 asshown in FIG. 17. In the presence of an electric field that is appliedin the Z direction (as shown at 156 in FIG. 18), the particlesagglomerate but because the particles are so long, they become entangledwith one another when agglomeration occurs. This results in theparticles not only providing electrical conductivity in the Z direction,but also providing electrical conductivity in the X and Y directions aswell due to the entangled mass of particles extending in the X and Ydirections as well as the Z direction as shown in FIG. 18.

Example 5

In accordance with a further example therefore, an adhesive mixtureincluding FLEXcon's V-95 acrylic adhesive, a polar material (ArquadHTL-8 sold by AkzoNobel, 20% solids on solids of the V-95 adhesive, and0.04% single walled semi-conductive carbon nanotubes (CNTs). The mixturewas provided in a 3% solids paste in a 72/28 solvent blend isopropylalcohol/n-butyl alcohol (sold by Southwest Nanotechnologies of 2501Technology Place, Norman, Okla. The mixture was sonicated for 30 minutesto evenly disperse the CNTs throughout the adhesive/arquad premixture.

The mixture was then coated, dried and cured as discussed above to a 2mil (50 micron) dried thickness. The adhesive composites were made andtested as discussed above. The results were that the pre-DOR test (asper EC12-2000-4.2.2.1) showed an impedance of 100 k Ohms. The DOR test(as per EC12-2000-4.2.2.4) was pass, and that the impedance postEC12-2000-4.2.2.1 was 5 K Ohms. The signal receptivity was tested as inExample 1 to be both 95% before and after DOR. The anisotropy test asdiscussed above with respect to Example 3, found that there was an X andY conductivity component to the composite post DOR. It is expected thatmore uniform istropic conductive coatings may be formed.

Example 6

As noted above, if the particle concentration in a pressure sensitiveadhesive is high enough to form a network in which particle-to-particlecontact is maintained then there is little chance that the adhesivecomponent is present in high enough concentrations to flow out to makesurface-to-surface contact between the substrates and an electrode,i.e., act as an adhesive. In a further example, to the adhesive materialof Sample 1 (the V-95 PSA and polar material) was added 25% by weight ofthe carbon particles of Sample 1. The composite was then coated anddried onto a polyester based siliconized release liner to a 2 mil (50micron) dry deposition. The resulting coating had substantially nomeasurable PSA properties (tack, peel, shear). An electricallyconductive network, however, had formed in the composite, and thiscomposite was found to have a DC resistance of about 100 Ohms bothbefore and after electrophoresis.

Those skilled in the art will appreciate that numerous modifications andvariations may be made to the above disclosed embodiments withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. An alternating electric field responsivebiomedical composite providing capacitive coupling through thecomposite, said biomedical composite comprising a binder material, apolar material that is substantially dispersed within said bindermaterial, and electrically conductive particles within the bindermaterial, wherein said polar material is responsive to the presence ofan alternating electric field, and wherein said electrically conductiveparticles are not of sufficient concentration to form a conductivenetwork through the composite.
 2. The composite of claim 1, wherein saidbinder material includes a polymeric material.
 3. The composite of claim1, wherein said electrically conductive particles include any of carbonpowder, flakes, granules or nanotubes.
 4. The composite of claim 3,wherein the carbon is in the form of graphite.
 5. The composite of claim1, wherein said electrically conductive particles have densities withinthe range of about 0.35 g/cm³ and about 1.20 g/cm³.
 6. The composite ofclaim 1, wherein said electrically conductive particles have densitieswithin the range of about 0.5 g/cm³ and about 1.0 g/cm³.
 7. Thecomposite of claim 1, wherein a surface energy of the conductiveparticles is higher than a surface energy of the binder material.
 8. Thecomposite of claim 1, wherein the conductive particles are randomlydistributed within the composite.
 9. The composite of claim 1, whereinthe conductive particles are provided in the composite in an orderedarrangement.
 10. The composite of claim 1, wherein the compositeincludes a conductive layer that is formed of carbon.
 11. The compositeof claim 1, wherein the composite includes a conductive layer having nosilver/silver chloride.
 12. The composite of claim 11, wherein thecomposite includes a plurality of connection types for electronicallycoupling to monitoring equipment.
 13. An alternating electric fieldresponsive composite providing capacitive coupling through the compositefor biomedical applications, said composite comprising a bindermaterial, a polar material that is substantially dispersed within saidbinder material, and electrically conductive particles within the bindermaterial, wherein said binder material and said polar material exhibitmutual molecular compatibility, and wherein said electrically conductiveparticles remain substantially isolated from one another within thebinder material.
 14. The composite of claim 13, wherein saidelectrically conductive particles include any of carbon powder, flakes,graphite granules or nanotubes.
 15. The composite of claim 13, whereinsaid electrically conductive particles have densities within the rangeof about 0.5 g/cm³ and about 1.0 g/cm³.
 16. The composite of claim 13,wherein a surface energy of the conductive particles is higher than asurface energy of the binder material.
 17. A method of providing aovercharge protection in an alternating electric field responsivecomposite, said method comprising the steps of providing a bindermaterial, adding a polar material to the binder material such that thepolar material becomes substantially dispersed within said bindermaterial and the binder material and the polar material exhibit mutualmolecular compatibility, and adding electrically conductive particles tothe binder material such that the electrically conductive particlesremain substantially isolated from one another within the bindermaterial unless and until an overcharge is applied that causes theelectrically conductive particles to undergo electrophoresis within thecomposite and form conductive paths through the composite.
 18. Themethod of claim 17, wherein said electrically conductive particles havedensities within the range of about 0.5 g/cm³ and about 1.0 g/cm³. 19.The method of claim 17, wherein a surface energy of the conductiveparticles is higher than a surface energy of the binder material.